Container for hot fill food packaging applications

ABSTRACT

A plastic container suitable for hot-fill food packaging applications is disclosed which is characterized by walls of proportionately decreasing thickness from the mouth of the container to a predetermined collapsible point. The wall thickness is designed such that the container walls will collapse, or deform, only at the collapsible point during cooling after hot-filling of food product or during transportation of the container between locations of varying altitudes and pressures. The container preferably collapses in the base area such that the collapse is not visible to the consumer and also the collapse does not affect stability of the container while in use or during loading and storage. The container of the invention is advantageous in that it requires less plastic material to form than other known hot-fill containers, and also can be formed out of any suitable food-grade plastic material or by any process.

FIELD OF THE INVENTION

The invention relates to a plastic container suitable for hot-fill foodpackaging applications, and a method of making the container. Thecontainer is manufactured in a manner such that the walls have asufficient strength to withstand collapse upon cooling of the containerafter sterilization, or during the transport of the filled containerbetween locations of varying altitude. A collapsible point, preferablythe bottom of the container, is manufactured to selectively deform tocompensate for the pressure differentials experienced between theinterior of the container and atmospheric pressure after hot-filling,and during transport of the container between locations of differentaltitudes. Also disclosed is a method of forming a container having aselectively deformable collapsible point, and walls that do not deform,after hot-filling and during other pressure changes.

BACKGROUND OF THE INVENTION

The storage of food products in plastic containers is well known.Further, it is known that food products can be hot-packed into plasticcontainers and then sealed to satisfy requirements for food products aswell as to provide extended shelf life.

Typically, a food product is packed in the container while the foodproduct is at sterilization temperature and then the container is fittedwith a sealed cover. An air space exists between the top level of thehot-packed food product and the seal. As the food product cools, thereduction in temperature causes a reduction in this air volume. Thisreduction in air volume results in a corresponding reduction in pressurewithin the container, and can cause the container to collapse due to thehead space remaining between the food product and the seal of thecontainer after sealing and cooling. In addition, any changes in thealtitude of the location of the packed container can cause similarexpansion and/or contraction of the container due to changing pressuredifferentials between the interior of the container and the atmosphericpressure at the different locations.

Such plastic containers must be designed to withstand contraction insidethe container caused by cooling of the hot-filled food product, as wellas those pressure changes that occur during transport of the plasticcontainers. Container deformation typically can occur at any location inthe container where the container wall cannot withstand the experiencedpressure differential between the inside of the container and theatmospheric pressure. Although container deformation does notnecessarily affect the sterility and stability of the packed foodproduct, consumers tend to shy away from such products based on theappearance that the containers are perhaps damaged or otherwise spoiled.In addition, container deformation causes the containers to be difficultto load and store due to the nonuniformity of the walls or bases. Also,container deformation can make a container unstable when used by theconsumer, when for example the base is deformed such that the containeris not steady when placed on a surface.

Many designs of plastic containers are known to fulfill theserequirements. One approach has been to design a plastic container havingwall design of sufficient rigidity to withstand the pressuredifferentials. For example, U.S. Pat. Nos. 4,318,882 and 4,497,855issued to Agrawal et. al. both titled “Method for Producing a CollapseResistant Polyester Container for Hot Fill Applications,” thedisclosures of which are hereby incorporated by reference in a mannerconsistent with this disclosure (“Agrawal”), disclose a polyestercontainer having at least one region that is thermoelasticallydeformable inwardly after the container is hot filled and sealed tooffset the pressure forces which tend to collapse the container as thecontents cool and create an internal vacuum. Agrawal discloses a processof forming this region by a two step molding process in which the regionis formed and heat set at a first position and then reformed outwardlyto a second position and cooled in that position.

U.S. Pat. Nos. 6,062,409 and 6,347,717 issued to Eberle both titled “HotFill Plastic Container Having Spaced Apart Ribs,” the disclosures ofwhich are hereby incorporated by reference in a manner consistent withthis disclosure (“Eberle I”), disclose a blow molded plastic containercomprising a plurality of vacuum panels having substantially archedupper and lower ends and vacuum panel reinforcement means in a series ofarched ribs. The design of the container minimizes the stress placed onthe corners of vacuum panels and resists flexing when the container isfilled with a hot liquid.

Similarly, U.S. Pat. Nos. 5,178,289 and 5,279,433 to Krishnakumar et.al. both titled “Panel Design for a Hot-Fillable Container,” thedisclosures of which are hereby incorporated by reference in a mannerconsistent with this disclosure (“Krishnakumar”), disclose a vacuumpanel design for a hot-fill container which resists the increase incontainer diameter which may occur during hot-filling or when thecontainer is dropped on a hard surface.

U.S. Pat. No. 5,337,909 issued to Vailliencourt and titled “Hot FillPlastic Container Having a Radial Reinforcement Rib,” the disclosure ofwhich is hereby incorporated by reference in a manner consistent withthis disclosure (“Vailliencourt”), discloses a container of a heat setmaterial having a plurality of elongated vertically oriented vacuumpanels in its sidewall and first and second circumferentially extendinginwardly directed reinforcement ribs which permit the center portions ofthe panels to flex inward during filling and sealing the container witha hot liquid but resisting deformation of the container sidewall. U.S.Pat. No. 5,341,946 also issued to Vailliencourt et. al. and titled “HotFill Plastic Container Having Reinforced Pressure and AbsorptionPanels,” the disclosure of which is hereby incorporated by reference ina manner consistent with this disclosure (“Vaillencourt et.al.”),discloses a container having a plurality of vertically oriented vacuumabsorption panels to prevent the sidewall from taking a permanent setdeflected inwardly.

Another approach is to use a specially designed base configuration toprovide stability to the container. For example, U.S. Pat. No. 5,005,716issued to Eberle titled “Polyester Container for Hot Fill Liquids,” thedisclosure of which is hereby incorporated by reference in a mannerconsistent with this disclosure (“Eberle II”), discloses a polyestercontainer having a base configuration including an outer circular ringdefining a support plane for the container with a central outwardlyconcave dome portion therein. The dome portion includes a number ofreinforcing rings formed along concentric tangent lines. Theconfiguration is designed to provide mechanical stability in response topositive and negative pressures within the container that tend to causedeformation of the container. Similarly, U.S. Pat. No. 4,993,567 issuedto Eberle Jr. titled “Involute Embossment Base Structure for Hot FillPET Container,” the disclosure of which is hereby incorporated byreference in a manner consistent with this disclosure (“Eberle Jr.”),discloses a base configuration for a blow molded plastic containerhaving a peripheral support ring that is generally concentric with thecontainer side walls and connected to a central dome structure. A numberof embossments raised around the central disk to resist deformationinduced by stresses incurred during hot filling of the container.

Another approach has been to design plastic containers with certaincrystalline structures that are less susceptible to deformation duringthe sterilization process. For example, U.S. Pat. No. 5,106,567 issuedto Demerest titled “Method for Producing a Heat Set Article ofThermoformed Polyethylene Terephthalate,” the disclosure of which ishereby incorporated by reference in a manner consistent with thisdisclosure (“Demerest”), discloses a article made of polyethyleneterephthalate (“PET”) having a degree of crystallinity of at least 20%giving the article more uniform and reproducible impact resistance overamorphous PET articles.

U.S. Pat. No. 4,582,665 issued to Jabarin titled “Method of MakingPolyethylene Terephthalate Articles,” the disclosure of which is herebyincorporated by reference in a manner consistent with this disclosure(“Jabarin”), discloses a method of making an oriented and heat setthermoformed article of PET. The sidewalls of the article have aspecific density, and the article is quenched after forming while underrestraint in order to limit volume shrinkage and to increase theonset-of-shrinkage temperature of the article.

Yet another approach includes a combination of container design andcontrolled treatment of the container after it has been filled. See,e.g., U.S. Pat. Nos. 4,642,968 and 4,667,454 to McHenry et. al. titled“Method Of Obtaining Acceptable Configuration Of A Plastic ContainerAfter Thermal Food Sterilization Process,” the disclosure of which ishereby incorporated by reference in a manner consistent with thisdisclosure (“McHenry”). McHenry discloses a method of obtaining anacceptable configuration of a thermally processed container packed withfood. The disclosure teaches that such a configuration can be obtainedby proper container design, by maintaining proper headspace of gases inthe container during thermal processing, proper pressure outside thecontainer during the cooking cycle and cooling cycle of the processand/or by controlled reforming of the bottom wall of the container. Inaddition, further improvements can be obtained by controlling thethermal history of the empty container.

U.S. Pat. No. 5,234,126 issued to Jonas et. al. titled “PlasticContainer,” the disclosure of which is hereby incorporated by referencein a manner consistent with this disclosure (“Jonas”), discloses aplastic container made in accordance with equations relating toreforming pressure and low fill equilibrium pressure giving thecontainer a unique bottom configuration which, independent of wallthickness, obviates paneling and other deformations of the containerduring sterilization.

Colombian Patent No. 25357 issued to Alberto, titled “Moving BottomContainer,” the disclosure of which is hereby incorporated by referencein a manner consistent with this disclosure, discloses a formed plasticcontainer having a base design that collapses to compensate for changesin pressure due to the effect of changes in height relative to sea-levelduring transport, storage and sale of the packed containers.

BRIEF SUMMARY OF THE INVENTION

The invention relates to a plastic container suitable for the packagingof hot-filled food products that can withstand the pressure changescaused by the cooling of the hot food product in the sealed container,and also during transport of the packed containers between locationshaving a pressure differential. The invention also relates to a methodof manufacture of the container. The container of the invention includeswalls designed by their thickness to have a collapsible point, whichcollapsible point is controllable by the manufacturing process.Preferably, the collapsible point is designed to be located in the baseof the container such that it is not visible to the consumer after itcollapses, and also is designed such that the stability of the containerwhen standing is not affected.

Where the collapsible point is designed to be located in the base of thecontainer, the thickness of the container walls are controlled such thatthe walls can withstand a designed pressure force without deformation,except for the collapsible point. This is done by controlling themanufacturing process to maintain the desired wall and bottom surfacethicknesses of the container. In the preferred embodiment, the wallthickness decreases proportionately from the area of the containersubstantially adjacent to the mouth, to the bottom of the container, andthe thickness of the bottom is less than the thickness of the walls.This is accomplished in the preferred forming process of thermoformingby controlling the dimensions of the mold and the counter-mold, andcontrolling the expulsion (ejection) pressure and the temperature of theplastic sheet during forming. In addition, the desired collapsible pointmay be further induced by a surface design that will facilitate thecollapse of that point at a lesser pressure differential that can bewithstood by other components of the container. The collapse of thecollapsible point reduces the head space between the food product andthe seal in the interior of the container, and then reduces oreliminates the probability that the walls of the container will alsocollapse, or otherwise deform.

The container of the invention can be made of any plastic materials thatare suitable for food packaging, and any forming method. For example,common plastic materials include polyethylene terephthalate (PET),polyester, ethylene, polystyrene, low density and high densitypolyethylene and polypropylene, or combinations of these materials.Additives may be included with the plastic materials to enhanceproperties desired in the final container. For example, additionalbarrier materials, such as polyvinyldichloride (“PVDC”),polyvinylchloride (“PVC”), or ethylene vinyl acetate-vinyl alcoholresins (“EVOH”) may be added to decrease permeability of the containerto such undesirable materials as oxygen and water vapor. Materials maybe added to adapt the optical properties. For example, materials may beadded to decrease light penetration into the container, or to increaseclarity of the container.

The container may be made by numerous known methods of forming, such asblow molding, thermoforming, extrusion, injection molding, blisterpackaging, vacuum forming or any other method of forming.

One object of the invention is to provide a plastic container suitablefor hot-fill food packaging that can withstand pressure differentialsincurred after hot-filling and transport to locales of varying altitudeswithout undesirable, or uncontrolled, deformation of the containerwalls.

It is another object of the invention to provide a plastic containersuitable for hot-fill food packaging that has a controlled collapsiblepoint such that any deformation of the container walls, and particularlythe bottom surface, does not affect the stability of the container whenit is placed on a surface for use.

It is another object of the invention to provide a plastic containersuitable for hot-fill food packaging that has a controlled collapsiblepoint such that any deformation of the container walls does not affectthe packing and handling characteristics of the container for transportand storage.

It is another object of the invention to provide a plastic containersuitable for hot-fill food packaging that has a controlled collapsiblepoint that can be prepared by the various forming processes.

Yet another object of the invention is to provide a plastic containersuitable for hot-fill food packaging having proportionately decreasingwall thickness from the mouth of the container to a predeterminedcollapsible point in the bottom surface of the container such that thecontainer does not collapse during or after hot-filling or duringtransportation except at the collapsible point.

It is another object of the invention to provide a plastic containersuitable for hot-fill food packaging that has a controlled collapsiblepoint such that the total amount of plastic used to manufacture the cupis reduced over containers of comparable size and application asmanufactured by other currently known processes.

It is another object of the invention to provide a method formanufacturing a plastic container suitable for hot-fill food packagingthat has walls of proportionally decreasing thickness and a controlledcollapsible point such that any deformation of the container walls donot affect the appeal of the container to the consumer.

These and other objects will become apparent to those of ordinary skillin the art through the description provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a schematic of a filled container manufactured according toprevious technology.

FIG. 1 b is a schematic of the bottom surface design of a filledcontainer manufactured according to previous technology.

FIG. 2 a is a schematic of a container manufactured according to oneembodiment of the invention.

FIG. 2 b is a schematic of the bottom surface design of a filledcontainer manufactured according to one embodiment of the invention.

FIG. 3 is a graphical representation of a typical heat treatment of aplastic sheet for solid phase plastic forming.

FIG. 4 is a flow scheme of the process of the invention formanufacturing a container according to one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The system and method of the invention relates to a container suitablefor hot-fill applications having walls that can withstand the pressuredifferentials experienced during and after sterilization, and alsoduring transport of the packed containers between locations having apressure differential. A selected collapsible point is designed into thecontainer, typically the bottom, that compensates for the pressuredifferentials experienced during the sterilization process and transportbetween locales of different pressures. The collapsible point may bedesigned such that the point of collapse, or deformity of the containersurface, may easily be hidden from exterior view by placement of asleeve over the container, or by other means. Alternatively, thedeformed surface may be designed such that it collapses into thecontainer itself, where it is hidden from view. In this embodiment, thecontainer walls adjacent to the bottom surface should be designed suchthat they do not collapse as well when the bottom surface collapses. Thebottom surface can be designed to have a surface design that enhancesits collapsing property, and reduce and/or eliminate any collapsing ofthe adjacent walls.

In the preferred embodiment of the invention, the walls of the containeras made by the thermoforming process can withstand pressuredifferentials between the interior of the sealed container and theatmospheric pressure of 12 psi, while the bottom of the containercollapses between about 2.5 psi to about 10 psi pressure differentialbetween the interior of the sealed container and the atmosphericpressure. In the preferred embodiment of the invention, the collapsedbottom surface will typically not be able to “pop out” again aftercollapsing into the container.

The collapsible point is preferably selected by designing the thicknessof the surface where the collapsible point is located to withstand theleast pressure differential between the interior of the container andthe atmospheric pressure, as compared to the other surfaces of thecontainer, such as the walls of the container. Once the collapse of thecollapsible point occurs, the head space inside the container betweenthe food product and the seal is reduced, making it unlikely that thewalls of the container will collapse or otherwise deform. In addition,the other walls should be designed to withstand pressure differentialsbetween the inside of the sealed container, both before and after thecollapsible point has collapsed, and the atmospheric pressure that maybe expected to occur after hot-filling and/or transport. One method ofdesigning the walls to have sufficient strength is by the thickness ofthe walls. The thickness of the container walls may be uniform, orpreferably they may gradually decrease from a point substantiallyadjacent to the mouth of the container to a point substantially adjacentto the point where the container walls meet the bottom surface. Thedecrease of the container wall thickness may be uniform, although thatis not a requirement. Additionally, or alternatively, the containerwalls may include a design that increases strength against collapse ordeformation, as known to those skilled in the art of container design.In the preferred embodiment of the invention, such a structural walldesign is included at the area of the container wall substantiallyadjacent to the bottom surface. This assists in preventing any bucklingof the walls in that area when the bottom collapses. In addition, thecontainer wall may include a decorative design that may or may not addstructural integrity to that surface.

If the container is manufactured by another method other thanthermoforming, such as injection molding, the container wall and bottomsurface thickness may be controlled by a mold design that allows thewalls to be of a thickness to withstand pressure differentials betweenthe inside of the sealed container, both before and after thecollapsible point has collapsed, and the atmospheric pressure that maybe expected to occur after hot-filling and/or transport, and the bottomsurface of the container to be the selectively collapsible point bymaking the bottom surface of lesser thickness than the walls.Alternatively, a surface other than the bottom surface can be designedby the mold to be the selectively collapsible point.

Preferably, the method of forming the container is by thermoforming, andspecifically solid phase pressure forming (“SPPF”), although othersuitable methods such as melt phase forming (“MPF”), blow molding,injection molding, blister packaging, vacuum forming and extrusion maybe used.

The plastics that may be used in forming include FDA approved food gradeacrylic, low density polyethylene (“LDPE”), high density polyethylene(“HDPE”), polystyrene (“PS”), polypropylene (“PP”), crystallinepolyester (“CPET”), polyethylene (“PE”) and combinations of these orother materials that are currently used to achieve barrier propertieslisted as “other” recycle code 7. Other suitable plastics may beselected that are in compliance with 21 C.F.R. § 177.1360 of the FDAregulations, and also 21 C.F.R. § 175.105, or regulations relating tofood grade plastics of the locale that the container is to be used.Alternatively, if the container is not to be used for food applications,the choice of starting raw materials is not so limited.

Additives may be included with the plastic. For example, materials toenhance barrier properties may be added, such as ethylene vinylacetate-vinyl alcohol copolymer (“EVOH”), PVDC, PVC, nitrile barrierresin or Nylon™. Barrier materials may be desirable in applicationswhere the designer needs to reduce the permeation of oxygen and watervapor into the container. For example, typically oxygen and water vaportransmission rates of less than about 5 cc/mil/100 in²/24hours/atmosphere at 23° C./73° F. and 75% RH is desirable for foodpackaging applications. Also, typically an adhesive material is includedin the plastic material. One skilled in the art of forming containersfor food packaging will be able to select a suitable adhesive, and levelof adhesive, to be used in the plastic for the forming of a containeraccording to the invention. Suitable adhesives include Antistatic745-2AS™ and AS 745-5AS™ available from Comai; Antioxidant CESA-STATPPARFB12020™ available from Clariant; AS ASPA-2485™ available fromShullman; and Antistatic 40390™ available from Ampacet. Additives mayalso include materials for aesthetic or other functional purposes, suchas clarity and blockage of light to the packed food product.

In thermoforming, sheet plastic is molded into the desired shape via thepressing of formers, or molds, into a sheet of heated plastic. Familiarproducts manufactured by thermoforming include yogurt pots and simpletrays. According to the preferred embodiment of the invention, theplastic used to form the container comprises a sheet of mixed EVOH, PPpolymer and adhesive. A suitable EVOH is EVAL™ J102B resin availablefrom Eval Company of America, located in Houston, Tex. EVAL™ J102Bcomprises approximately 32 mol % ethylene, with a Melt Index (“MI”) ofapproximately 2.0 g/10 min @ 190° C. and 2160 g, and 4.9 g/10 min @ 210°C., 2160 g, according to ASTM D1238. The density is approximately 1.17g/cc according to ASTM D1505. The melting point and crystallizationtemperature, as measured by differential scanning calorimetry, isapproximately 183° C. and 161° C., respectively. The glass transitionpoint, as measured by dynamic viscoelasticity, is approximately 69° C.The oxygen permeability, as measured by ASTM D1434, is approximately0.03-14 cc/mil/100 in²/24 hrs/atm @ 65% relative humidity and 68° F.,and 1.8×10⁻¹⁴ cc/cm/cm²/sec/Hg @ 65% relative humidity and 20° C. Thewater vapor transmission rate, according to ASTM E96-E, is approximately3.8 g/mil/100 in²/24 hrs @ 90% RH and 100° F., and approximately 50 g/30μ/m²/24 hrs @ 90% RH and 40° C. The gloss at 45° is 85 as measured byASTM D2457. The haze is approximately 1.9% as measured by ASTM D1003.The ultimate tensile strength is approximately 8200 psi and the ultimateelongation is approximately 270%, as measured by ASTM D882.

A suitable PP is PROPILCO™ 03H96, available from Polipropileno delCaribe S.A. of Bogota, Colombia. The melt flow of PROPILCO™ 03H96 isapproximately 3 g/10 min, as measured by ASTM D123 at 230° C. and 2.16kg. The tensile yield strength is approximately 5300 ps, or 36.5 Mp andthe tensile yield elongation is approximately 9.3, as measured at 50mm/min by ASTM D638. The Rockwell hardness is approximately 102 asmeasured by D 785.

Preferably, the composition of the plastic sheet in the preferredembodiment is approximately 0% to approximately 15% by volume EVOH, andapproximately 80% to approximately 100% by volume PP, more preferablyapproximately 1% to approximately 7% by volume EVOH, and approximately85% to approximately 95% by volume PP, and even more preferablyapproximately 1% to approximately 5% by volume EVOH, and approximately90% to approximately 95% by volume PP. Additionally, adhesive may beincluded in amounts as determined by those skilled in the art to formthe container.

The plastic sheet is typically formed by extrusion, or coextrusion wherecombinations of materials may be used to form the plastic sheet.Typically, the various materials are extruded to form a multilayerplastic sheet. For example, in the preferred embodiment of theinvention, the plastic sheet comprises five layers of PP, adhesive,EVOH, adhesive and PP. However, other additives may be added togetherduring extrusion for use in a single layer. Those skilled in the art ofthermoforming will be able to select suitable conditions for forming theplastic sheet according to the desired properties of the finishedcontainer.

After the sheet of plastic that will be used to form the container isformed, the sheet is preferably preheated to a substantially uniformtemperature distribution using methods and equipment known to thoseskilled in the art of thermoforming. In the preferred embodiment of theinvention, an Illig 50K™ deep draw cup forming thermoforming machine isused having a forming area of about 500×280 mm. The Illig 50K™ isavailable from ADOLF ILLIG Maschinenbau GmbH & Co. KG located inHeibronn, Germany. The Illig 50K™ has five (5) heaters, each heaterhaving three zones. In the preferred embodiment, the sheet is heated bya succession of a plurality of heaters comprising upper and lowerheaters. The heaters form a “tunnel” or “oven” through which the plasticsheet passes on a belt conveyor. The heaters heat the plastic sheet toits VICAT temperature according to methods and processes known to thoseskilled in the art. Any heating equipment or method can be used so longas it achieves a substantially uniform temperature distributionthroughout the sheet. Of course, for different forming methods, othertemperatures may be necessary. For example, for injection molding, theplastic material must be completely melted to pour into the mold.

Typically during thermoforming the plastic sheet undergoes one or moreheating and cooling cycles. The actual thermoforming step comprisesfitting a plug into a mold to form the desired dimensions and shape ofthe container. After the container is formed, it cools almostimmediately and the formed containers are typically stacked directly outof the thermoformer.

Once the container is formed, it may be used for hot-fill foodapplications. The thickness of the walls of the container are sufficientto withstand the pressure differences experienced during the cooling ofthe hot-filled food product, and yet the thickness of the bottom of thecontainer allows the container to selectively collapse at that point,where the deformation remains unnoticed by the consumer or ultimatepurchaser of the container. Similarly, the deformation point may bedesigned to be located at a different location in the container, and theartisan skilled in the art of the selected forming process may adjustthe forming conditions appropriately to form the desired collapsiblepoint. The advantage of designing the collapsible point at the bottom ofthe container is that the container may then be placed in a decorativeouter sleeve that shields the collapsed bottom from view. Alternatively,and preferably, the bottom of the container is designed with a surfacedesign that allows mainly the center area of the bottom to collapse,such that the edges of the bottom surface that do not collapse providestability for the container.

Turning now to the figures, FIG. 1 a is a schematic of a filledcontainer manufactured according to previous technology. FIG. 1 adepicts a container 10 having a 11, a mouth 12, an inner chamber 14, abottom 15 and a side wall 16. The inner chamber 14 is filled with a foodmaterial to level 13 that is hot-filled into the container. The sidewalls 16 are designed to withstand the pressure differentials betweenthe interior of the sealed container and atmospheric pressure. Thebottom 15 is substantially level following hot-filling and cooldown ofthe hot-filled food product.

FIG. 1 b depicts the bottom surface of the container of FIG. 1 a havinga bottom surface design 17. Typically, such a bottom surface design 17is designed to withstand the pressure differentials between the interiorof the sealed container after hot-filling and atmospheric pressure.

A container manufactured according to the invention is depicted in FIG.2 a. The container 10 includes a mouth 11, an inner chamber 14, a bottom15 a, b and walls 16. A food product is hot-filled in the inner chamber14 to a level 13. The lower area of the walls 16 substantially adjacentto the bottom 15 a, 15 b may optionally, and preferably, include adesign 18 that may provide structural enhancements to that area of thewall 16 given its close location to the collapsible bottom 15 a, 15 b.This design 18 may also provide aesthetic enhancements, or may provideboth aesthetic and structural enhancements to the container 10.

In the container manufactured according to the invention, the bottom 15a is not necessarily level after manufacture, although the bottom 15 amay be level. After hot-filling and cooling of the food product, thebottom 15 b of the container selectively deforms by collapsing inwardtoward the inner chamber 14. When this occurs, any head space that hadbeen present between the level of the packed food 13 and the top of thecontainer is reduced and/or substantially eliminated. Notably, the walls16 remain substantially uniformly tapered from the mouth 11 to thebottom 15 b, and no deformation occurs. This is due to the increasedwall thickness of the walls 16 over the bottom 15 a, b. The wallthickness of the walls 16 and bottom 15 a, b have been designed andmanufactured to allow only for deformation of the bottom 15 a, b aftercooling of the hot-filled food product. This structural integrity may beenhanced by design 18.

The bottom 15 a, b preferably is designed to have a surface design 17,shown in FIG. 2 b, that further enhances the propensity of the bottom 15a, b to collapse selectively over the walls 16. In addition, the surfacedesign 17 is situated such that only the portion of the bottom 15 a, bhaving the surface design 17 actually collapses, permitting theuncollapsed outer edges of the bottom 19 to shield the collapsed bottomsurface 15 b from view, so that the container does not appear to include“deformities” from the perspective of a consumer. In addition, theuncollapsed outer edges of the bottom 19 also provide stability to thecontainer when standing upright.

The desired wall thickness can be determined by methods known to thoseskilled in the art of manufacturing containers for hot-fill food packingapplications by calculating the pressure within the interior of thecontainer after hot-filling and subsequent cooling of the food product.The container of the invention may also be designed to selectivelydeform at the bottom 15 and not the walls 16 for containers that arehot-filled at locales of higher elevation, and then transported tolocales of lower elevation. The container of the invention may also bedesigned to withstand pressure changes incurred during otherapplications, such as transport of the containers in non-pressurizedairplane cargo holds.

FIG. 3 provides a graphical representation of a common heat treatmentfor plastic sheets in preparation for thermoforming. This graphicalrepresentation is exemplary only, and is not intended to limit theinvention in any manner. FIG. 3 demonstrates that those skilled in theart of thermoforming typically follow a pattern of rapidly increasingthe temperature of the plastic sheet to be used for forming to atemperature higher than the VICAT temperature, but lower than themelting temperature. Then, the temperature is allowed to slowly andslightly cool to the desired VICAT temperature at which time the productis formed by punching the plug into the mold in the thermoformer. Thismethod of raising the temperature of the plastic sheet to the VICATtemperature may be used in the invention, although any other method thatproduces a desirable thermoformed product may also be used.

A schematic of the thermoforming process of the preferred embodimentinvention is depicted in FIG. 4. At Step 410, the plastic sheet thatwill be used in the thermoforming process is prepared. Typically, thesheet is prepared by extrusion, or coextrusion, according to principlesand methods known to those skilled in the art of plastics. Thecomposition of the plastic sheet is selected by the designer of thefinal product to include the clarity and barrier properties desired. InStep 420, an optional pretreatment of the plastic sheet may take place,if necessary, prior to thermoforming. Again, the necessity andconditions of the pretreatment step are selected by the designer of thefinal product who is acquainted with the thermoforming of the selectedplastic sheet composition. In Step 430, the plastic sheet is subjectedto a heat treatment to achieve the VICAT temperature of the plasticmaterials in the plastic sheet. In Step 440, the container is punchedfrom the plastic sheet by the molds of the thermoforming machine. InStep 450, the formed container is cooled and stacked where it may beused for hot-filling of food products.

Although the container may be any dimension desired by the designer, onecommon food packaging application is a four (4) ounce cup. A typicalfour ounce cup made according to the preferred embodiment of theinvention weighs approximately 4.8 grams. The wall dimensions range fromabout 0.71 mm adjacent to the mouth of the cup, then proportionatelydecreasing to about 0.22 to about 0.34 mm or an average of 0.28 mm atthe wall of the cup substantially adjacent to the bottom of the cup. Thebottom of the cup is on the average about 0.16 mm thick. A typical cupis approximately 79 mm wide, and approximately 52 mm in height.

One advantage of the container of the invention is that a container of acertain size may be manufactured using less forming material thanpreviously known containers of comparable size, resulting in costsavings in raw materials. For example, the four ounce cup previouslydescribed weighs about 4.8 g. In contrast, four ounce containersmanufactured using currently known techniques range from about 6.5 toabout 6.7 grams. The raw material savings in the container of theinvention, for a comparable sized container, can result in a savings ofover 35% up to about 50% in raw material costs. Further cost savings maybe realized if raw material costs increase.

This savings is realized because the containers previously producedinclude walls throughout the cup of sufficient thickness to preventcollapse at any point. By designing and manufacturing a container with aselectively collapsible bottom by the system and method of theinvention, the manufacturer can realize significant raw material costsavings while not experiencing increased manufacturing costs, when usingthe same forming process.

EXAMPLES

The following example is provided as a further description of oneembodiment of the invention, and is not intended to be limiting.

A four ounce cup was made according to the invention that corresponds tothe cup depicted in FIG. 1 by the thermoforming process. The place ofmanufacture was Medellin, Colombia, having an altitude above sea levelof about 6500 feet. Note that the forming conditions should be adjustedaccording to the ambient conditions of the location of the manufacturingfacility.

A starting plastic multilayer sheet comprising about 70-80 volume %EVAL™ J102B resin, about 20-40 volume % PROPILCO™ 03H96 PP and about15-20 volume % Comai 745-2AS™ adhesive, based on a total thickness ofthe multilayer sheet of about 1.02 mm, was formed by coextrusion. Theapproximately 49 mm wide sheet was continuously fed at a rate sufficientto mold four ounce cups, ten cups at a time, 14 cycles per minute. Theplastic sheet was fed into an Illig 50K™ thermoforming machine where thefirst zone heaters were set at about 350° C./425° C./300° C.; the secondzone heaters were set at about 335° C./310° C./370° C.; and the firstheater of the third zone was set at about 335° C. The cups were thenformed from the multilayer plastic sheet using a mold refrigerated toabout 13° C., and a countermold, also known as plug-assist, refrigeratedto about 11° C. The cups were ejected by air pressurized to about 5 psifrom a distance of about 95 mm from the cutting tool.

Each container was approximately four ounces, and about 79 mm in widthand about 52 mm in height, with a weight of about 4.8 grams. The walldimensions range from about 0.71 mm adjacent to the mouth of the cup,then proportionately decreasing to about 0.22 to about 0.34 mm or anaverage of 0.28 mm at the wall of the cup substantially adjacent to thebottom of the cup. The bottom of the cup is on the average about 0.16 mmthick. A typical cup is approximately 79 mm wide, and approximately 52mm in height. The wall thickness was determined to provide sufficientrigidity to withstand pressure differences between the interior of thecontainer and the atmospheric pressure of up to 12 psi. The bottom ofthe container began to collapse at a pressure differential of about 2.5psi, and complete collapse was observed at about 10 psi pressuredifferential. These pressure differentials have been tested to meet orexceed the pressure differentials experienced by cups used for hot-fillfood product packaging under actual conditions.

The foregoing embodiments have been presented for the purpose ofillustration and description only and are not to be construed aslimiting the scope of the invention in any way. The scope of theinvention is to be determined from the claims appended hereto.

1. A plastic container, comprising: a mouth; a bottom surface; and acontainer wall between the mouth and the bottom surface, wherein one ofthe bottom surface or the container wall flexes inward into the cavityof the plastic container; wherein further the inward flexing of thebottom surface of the container wall reduces a pressure differentialbetween the inside of the container and atmospheric pressure when eitherthe container is hot-filled with food product or when the container istransported from a locale of lower atmospheric pressure to higheratmospheric pressure; and wherein further the non-flexing surfacemaintains the same form from prior to hot-filling or transport.
 2. Theplastic container of claim 1, wherein the thickness of the containerwalls decreases from a point substantially at the mouth to a pointsubstantially at the bottom surface.
 3. The plastic container of claim1, wherein the bottom surface flexes inward into the container cavity.4. The plastic container of claim 3, wherein the circumference of themouth is greater than the circumference of the bottom surface.
 5. Theplastic container of claim 4, wherein the plastic comprises a plasticsuitable for solid phase pressure forming.
 6. The plastic container ofclaim 5, wherein the plastic further comprises polypropylene.
 7. Theplastic container of claim 6, wherein the plastic further comprises abarrier enhancement agent.
 8. The plastic container of claim 7, whereinthe barrier enhancement agent comprises ethylene vinyl acetate-vinylalcohol.
 9. The plastic container of claim 8, wherein the plasticfurther comprises an adhesive suitable for solid phase pressure forming.10. The plastic container of claim 9, wherein the adhesive comprises anantioxidant
 11. The plastic container of claim 5, wherein the plasticcontainer is formed from a plastic sheet comprising up to about 15volume % ethylene vinyl acetate-vinyl alcohol, about 80 to about 90volume % polypropylene and about 15 to about 20 volume % adhesive. 12.The plastic container of claim 1, wherein the plastic container isformed from a plastic sheet having one or more layers, and whereinfurther the thickness of the container walls are about 70-80 volume % ofthe thickness of the plastic sheet at a location substantially adjacentto the container mouth and about 20-40 volume % of the sheet at alocation substantially adjacent to the bottom surface, and the thicknessof the bottom surface is about 15-20 volume % of the thickness of theplastic sheet.
 13. The plastic container of claim 12, wherein thecontainer wall thickness uniformly decreases from a locationsubstantially adjacent to the container mouth to a point substantiallyadjacent to the bottom surface.
 14. The plastic container of claim 13,wherein the container walls are about 0.7 mm thick at a locationsubstantially adjacent to the container mouth and about 0.28 mm thick ata point substantially adjacent to the bottom surface, and the thicknessof the bottom surface is about 0.16 mm.
 15. A method for forming aplastic container, comprising: selecting at least one polymer for aplastic container; and forming the plastic container; wherein theplastic container comprises: a mouth; a bottom surface; and a containerwall between the mouth and the bottom surface, wherein one of the bottomsurface or the container wall flexes inward into the cavity of theplastic container; wherein further the inward flexing of the bottomsurface of the container wall reduces a pressure differential betweenthe inside of the container and atmospheric pressure when either thecontainer is hot-filled with food product or when the container istransported from a locale of lower atmospheric pressure to higheratmospheric pressure; and wherein further the non-flexing surfacemaintains the same form from prior to hot-filling or transport.
 16. Themethod of claim 15, wherein forming the container comprises: heating theplastic sheet to its VICAT temperature; and thermoforming the container.17. The method of claim 15, wherein forming the container comprisesextrusion, vacuum forming, injection molding, blister packaging, meltphase forming or blow molding.
 18. A method of manufacturing a plasticcontainer with a selectively deformable surface, comprising: selectingat least one polymer; heating the at least one polymer to its VICATtemperature; and thermoforming a container from the heated polymer;wherein the plastic container comprises: a mouth; a bottom surface; anda container wall between the mouth and the bottom surface, wherein oneof the bottom surface or the container wall flexes inward into thecavity of the plastic container; wherein further the inward flexing ofthe bottom surface of the container wall reduces a pressure differentialbetween the inside of the container and atmospheric pressure when eitherthe container is hot-filled with food product or when the container istransported from a locale of lower atmospheric pressure to higheratmospheric pressure; and wherein further the non-flexing surfacemaintains the same form from prior to hot-filling or transport.
 19. Themethod of claim 18, wherein the thickness of the container wallsdecreases from a point substantially at the mouth to a pointsubstantially at the bottom surface.
 20. The method of claim 18, whereinthe bottom surface flexes inward into the container cavity.
 21. Themethod of claim 20, wherein the circumference of the mouth is greaterthan the circumference of the bottom surface.
 22. The method of claim21, wherein the plastic comprises a plastic suitable for solid phasepressure forming.
 23. The method of claim 22, wherein the plasticfurther comprises polypropylene.
 24. The method of claim 23, wherein theplastic further comprises a barrier enhancement agent.
 25. The method ofclaim 24, wherein the barrier enhancement agent comprises ethylene vinylacetate-vinyl alcohol.
 26. The plastic container of claim 25, whereinthe plastic further comprises an adhesive suitable for solid phasepressure forming.
 27. The plastic container of claim 26, wherein theadhesive comprises an antioxidant
 28. The plastic container of claim 22,wherein the plastic container is formed from a plastic sheet comprisingup to about 15 volume % ethylene vinyl acetate-vinyl alcohol, about 80to about 90 volume % polypropylene and about 15 to about 20 volume %adhesive.
 29. The plastic container of claim 18, wherein the plasticcontainer is formed from a plastic sheet having one or more layers, andwherein further the thickness of the container walls are about 70-80volume % of the thickness of the plastic sheet at a locationsubstantially adjacent to the container mouth and about 20-40 volume %of the sheet at a location substantially adjacent to the bottom surface,and the thickness of the bottom surface is about 15-20 volume % of thethickness of the plastic sheet.
 30. The plastic container of claim 29,wherein the container wall thickness uniformly decreases from a locationsubstantially adjacent to the container mouth to a point substantiallyadjacent to the bottom surface.
 31. The plastic container of claim 30,wherein the container walls are about 0.7 mm thick at a locationsubstantially adjacent to the container mouth and about 0.28 mm thick ata point substantially adjacent to the bottom surface, and the thicknessof the bottom surface is about 0.16 mm.